The invention relates to an axial fan, especially for the cooler or radiator of a water cooled internal combustion engine for commercial vehicles, and of the type which includes a hub with impeller or ventilator blades and an air guide frame (shroud) or device surrounding the blades which expands or enlarges in the air exit region.
Axial fans of this general kind are known (see German published application-DE-AS 2411225). Such cooling fans as a rule generally have very high pressure losses to overcome and so are strongly throttled. Most of the cooling fans that are on the market today for commercial vehicles operate in a region from .tau.=0.07 to 0.15, wherein .tau. is a so called throttle number corresponding to a ratio of the dynamic pressure, taken at the impeller ring surface, divided by the total pressure increase, whereby the total pressure increase is the combination of the dynamic pressure and the static pressure. With a strong throttling of this kind, therefore with high resistant pressure and thereby resulting small volume flow, the stream or flow separates at the hub. There then results no pure axial extending flow in the region of the fan blades, but rather a so called half axial flow by which the air is accelerated diagonally toward the outside, also if no combustion motor is arranged behind the fan. The known fans of this kind produce a relatively high noise level during operation, which noise level can be held at a lower level through the widening of the air guide frame in the direction of the exit region, as compared to arrangements without such a widening. The present invention is based upon the proposition to facilitate a further reduction of the noise level, without however changing the overall construction.
The invention comprises an arrangement wherein the expansion or widening of the air guide frame begins already in the region of the entrance edge of the fan blades and exhibits an inner contour which conforms to the outer contour of the adjacent fan blades. With this form or shape, a clear reduction in the noise level in the operating or working point of the fan is achieved without however adversely influencing the fan output.
It is advantageous if the axial length of the air guide structure is at least 40% of the axial length of the fan blades at their outer diameter. Advantageously the length of the air guide apparatus is, however, selected to be still larger. A practical embodiment of the air guide apparatus is provided with a circular or similar curved expansion or widening part, connected with a conical diffuser part at its exit side. This embodiment provides good results with respect to the noise level and with respect to the fan performance.
For the design arrangement of the air guide apparatus there are two advantageous possibilities according to the invention. The air guide structure can be constructed as a fixed fan frame which surrounds the rotating fan blades in a very simple manner. However the air guide structure can also be formed in part from the hub and the fan blade of an existing fan wheel, in the form of a guide ring fixedly connected with the ends of the fan blades. Through these measures there results a clear lowering of the sound level as compared to the known construction forms wherein the fan wheel includes an outer ring rotating therewith (see German unexamined published application DE-OS-2826697). It is advantageous if the entrance edge of the guide ring extends over the fan entrance edge against the flow direction and forms an axially extending ring part. This ring part can then be used in an especially advantageous manner so that it extends into a fixed nozzle which does not rotate and which is arranged adjacent the entrance edge of the guide ring to form an entrance opening. Thereby the main stream is stabilized in the direction of the inlet slot air which also enters between the nozzle and the rotating guide ring. A further improvement can be achieved in that the entrance nozzle is expanded in a curved form from its smallest cross section in the flow direction. The slot-and main streams are thus guided tangentially onto the expanding section of the rotating guide ring to therefore improve the introduction of the flow onto the expansion. Finally the noise conditions and the efficiency of the fan can be still further improved if the entrance nozzle encompasses the rotating guide ring from the outside. The slot air thereby travels through a 180.degree. turn-around with corresponding throttling so that the slot losses become small. This results not only in an improvement in efficiency, but rather also in a reduction of the sound level.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, a single embodiment in accordance with the present invention.